1. Field of the Invention
The present invention relates to an optical fiber drawing furnace which can draw an optical fiber with small non-circularity.
2. Description of the Related Art
An optical fiber is obtained by heating a perform for optical fiber in an optical fiber drawing furnace to melt and by drawing the molten preform for optical fiber from the lower end thereof. More concretely, as disclosed in Japanese Patent Application Laid-open No. 8233/1988, the optical fiber drawn from the lower end portion of the optical fiber preform is instantly applied a resin protecting layer, such as ultraviolet ray curing resin on the outer periphery by a resin applicator. Furthermore, by passing through a resin curing apparatus, the resin protection layer is cared. Thereafter, the optical fiber is taken up by a take-up machine. On the other hand, by a diameter measuring device provided between the drawing furnace and the resin applicator, a diameter of the optical fiber is measured. Then, a drawing speed of the optical fiber to be drawn from the optical fiber preform is adjusted so that the diameter of the drawn optical fiber is held constant.
On the other hand, when the non-circularity of the optical fiber is large, a diameter of the hole of the optical fiber connector ferrule for setting the optical fiber has to be made greater. When the hole diameter for setting the optical fiber is made greater, an offset between an axis of the hole and an axis of the optical fiber becomes greater to possible cause large connection loss. Similarly, upon mutual connection by mating a pair of optical fibers utilizing a V-shaped groove formed in a positioning block, radius of the optical fiber may fluctuate at the portion contacting with the V-shaped groove. Therefore, the non-circularity of the optical fiber should be a cause of offset of axis upon connection or connection loss.
In order to derive the non-circularity, the diameters of the optical fiber are measured at a plurality of positions in the circumferential direction. With taking the largest diameter among the measured diameters as xe2x80x9clonger diameterxe2x80x9d and the smallest diameter as xe2x80x9cshorter diameterxe2x80x9d. Then, the non-circularity can be expressed by (longer diameter-shorter diameter)/average diameter. In general, assuming that the maximum diameter of the objective circularity is D1 and the minimum diameter is D2, the non-circularity xcex5 is expressed by the following equation.
xcex5={2xc3x97(D1xe2x88x92D2)/(D1+D2)}xc3x97100(%)
Conventionally, in order to reduce the non-circularity to zero percent as close as possible, the lower end of the optical fiber preform is matched with the center of the furnace to uniformly heat the optical fiber preform along the circumferential direction thereof. On the other hand, as disclosed in Japanese Patent Application Laid-open No. 96042/1989, there has been proposed a technology for solving a problem of temperature fluctuation by rotating a muffle tube. Also, as disclosed in Japanese Patent Application Laid-open No. 227837/1994, there has been a proposal for reducing non-circularity by restricting temperature fluctuation along the circumferential direction of the heater by extending a pair of electrode connecting portions opposing with an angle of 180 degrees interval to the outer peripheral well of the furnace body and connecting electrodes to the tip ends of the electrode connecting portions.
One example of sectional structure of such conventional drawing furnace is shown in FIG. 9 and the external view of the heater of the conventional drawing furnace is illustrated in FIG. 10. As shown, in a center portion of a furnace body 102, a cylindrical muffle tube 103 is assembled. Also, between the center portion of the furnace body 102 and the muffle tube 103, a heater 104 of cylindrical configuration as a whole is diagnosed. Surrounding the heater 104, a heat insulator 101 is set. By the heater 104, a lower end portion of the optical fiber preform (not shown) supplied into the muffle tube 103 is heated for melting. Then, the molten portion of the optical fiber preform is drawn from the lower end of the furnace body 102 as an optical fiber.
The heater 104 has a heating portion 105 meandering in vertical direction and formed in to a cylindrical configuration as a whole, and a set of two electrode connecting portions 106 and 107 mutually offset with an angle of 180 degrees interval in circumferential direction of the heating portion 105. These one set of the electrode connecting portions 106 and 107 are extended from the upper end of the heating portion 105 to radially project outwardly. To these one set of the electrode connecting portions 106 and 107, one set of electrodes 108 and 109 lead outside of the furnace body 102 are connected via connecting members 110. These electrodes 108 and 109 are connected to a single phase AC power source 113 via a transformer 111 and a power control unit 112. A current supplied from the single phase AC power source 113 via the power control unit 112 passes through the heating portion 105 via the connecting member 110 and one of the electrode connecting portion 106 from one of the electrode 100 and flows to the connecting member 110 and the other electrode 109 from the other electrode connecting portion 107, or flows in opposite direction to heat the heating portion 105
In the recent years, associating with lowering of cost of the optical fiber, the optical fiber preform is becoming into greater diameter and drawing speed is becoming higher. On the other hand, the drawing furnace per se inherently cause certain non uniformity of the temperature in the circumferential direction due to presence of electrodes, cooling water flow and so forth, for example. Non-uniformity of temperature distribution in the circumferential direction becomes more significant is greater diameter of the optical fiber preform than that smaller diameter to cause more significant temperature fluctuation within the optical fiber preform. Furthermore, when the optical fiber is drawn at higher speed than that in the prior art, a period to pass a neck down portion becomes shorter where the optical fiber preform is molten and reduced in diameter. This results in difficulty of reduction of the temperature fluctuation to increase tendency of increasing of non-circularity.
On the other hand, in case of the method, in which the muffle tube is rotated as disclosed in Japanese Patent Application Laid-open No. 96042/1989, the temperature fluctuation of the optical fiber preform is advantageously reduced to permit reduction of the non-circularity of the optical fiber. However, on the other hand, by rotation of the muffle tube, flow of a gas within the furnace is disturbed to make fluctuation of the diameter of the optical fiber greater. As a result, upon connection by means of an optical connector or by mutually melting the connecting ends, connection loss can be increased.
Furthermore, the method to extend a pair of electrode connecting portions to the outer periphery of the furnace body as disclosed in Japanese Patent Application Laid-open No. 227837/1994, is advantageous in that the temperature fluctuation in the circumferential direction of the heater can be improved in comparison with the prior art. However, there still present a difference of temperature between the atmosphere in the vicinity of the electrode connecting portions opposing with an angle of 180 degrees interval and the atmosphere in the direction perpendicular to the opposing direction of the electrode connecting portions to possible make cross section of the obtained optical fiber somewhat elliptic.
It is therefore an object of the present invention to provide an optical fiber drawing furnace which makes it possible to draw an optical fiber with smaller non-circularity.
According to the first aspect of the present invention, an optical fiber drawing furnace comprises:
a muffle tube, in which an optical fiber preform is supplied;
a heater surrounding the muffle tube;
a plurality of electrode connecting portion extending from the heater, and connecting to an electric power source via a plurality of electrodes; and
unifying means for unifying the temperature distribution along the circumferenetial direction of the heater.
With the first aspect of the present invention, since the unifying means is provided, the temperature distribution along the circumferential direction of the heater can be made uniform. As a result, non-circularity of the optical fiber can be made smaller in comparison with the optical fiber drawn by the conventional drawing furnace. Therefore, high quality optical fiber with lesser connection loss can be obtained.
According to the second aspect of the invention, an optical fiber drawing furnace comprises:
a muffle tube, in which an optical fiber preform is supplied;
an inner heater surrounding the muffle tube;
a pair of inner electrode connecting portions extending from the inner heater, arranging in opposition to each other across the inner heater and connecting to a power source via a pair of electrodes,
an outer heater surrounding the inner heater, and
a pair of outer electrode connecting portions extending from the outer heater, arranging in opposition to each other across the outer heater in a direction perpendicular to opposing direction of the pair of inner electrode connecting portions and connecting to the power source via a pair of electrodes.
In the second aspect of the invention, since the opposing direction of a pair of electrode connecting portions projected from the inner heater and the opposing direction of a pair of electrode connecting portions projected from the outer heater are intersecting to each other, the temperature distribution along circumference of the muffle tube can be unified by the inner and outer heaters. As a result, it becomes possible to make the non-circularity of the optical fiber smaller than that in the prior art. Thus, a high quality optical fiber with small connection loss can be obtained.
In either aspect of the invention, the lower end of the optical fiber preform is heated by the heater and softened. By drawing the softened portion, the optical fiber is drawn continuously.
In the first aspect of the optical fiber drawing furnace according to the invention, the unifying means varying cross sectional area of the current path of the heater along the circumferential direction of the heater, or varying the path length in the longitudinal direction of the heater along the circumferential direction may be employed.
On the other hand, the unifying means may include the electrode in number of integer multiple of two arranged along circumferential direction of the heater with substantially equal intervals, and the electrode connecting portions in number of integer multiple of two but not smaller than four arranged along circumferential direction of the heater with substantially equal intervals. In such case, it is preferred that the power source is a single phase AC power source. Similarly, the unifying means may include the electrodes in number of integer multiple of three arranged along circumferential direction of the heater with substantially equal intervals, and the electrode connecting portions in number of integer multiple of three arranged along circumferential direction of the heater with substantially equal intervals, and, in such case, the power source may be a three phase AC power source.
The plurality of electrode connecting portions may be respectively connected with the electrodes. In such case, it is preferred that the unifying means includes two of the electrodes, the electrode connecting portions in number of integer multiple of two but not smaller than four, and connecting elements respective connecting half in number of the electrode connecting portions, the connecting elements being respectively connected to the electrodes
When the electrode connecting portions in greater number than that of the electrodes are provided, or when three or more electrodes are provided, temperature distribution along the circumferential direction of the heater can be made further uniform.
Furthermore, it is possible that resistance values of a plurality of current paths from one of the electrode to the other of electrode are set to be equal to each other. For this purpose, it is effective to form the heater of graphite. In this case, temperature distribution along the circumferential direction of the heater can made further uniform.
In the second aspect of the optical fiber drawing furnace, it is effective to vary cross sectional area of a current path for the heater along circumferential direction of the heater or to vary a path length in a longitudinal direction of the heater along the circumferential direction.